Jeans-Jeffreys tidal hypothesis

Schematic representation of the Jeans-Jeffreys tidal
hypothesis. (a) A tidal bulge is induced. (b) A filament of material
is drawn out in which condensations form. (c) The produced protoplanets
orbit the Sun with high eccentricities. Image credit: Stephen Oxley,
doctoral
thesis, Univ. of York, 1999

As a result of a detailed mathematical analysis, Jeans concluded in 1916
that the tidal interaction between the Sun and a passing star would raise
tides on the Sun resulting in the loss of a single cigar-shaped filament
of hot gas, rather than separate streams of gas as in the Chamberlin and
Moulton scenario. This hot gas would then condense directly into the planets
instead of going through a planetesimal
stage. The central section of the "cigar" would give rise to the largest
planets – Jupiter and Saturn
– while the tapering ends would provide the substance for the smaller
worlds.

This model had important repercussions for the possibility of life elsewhere
in the universe because if planetary systems came about only as a result
of freak stellar encounters, there would be relatively few extrasolar worlds
to provide biological platforms. In his 1923 lecture "The Nebular Hypothesis
and Modern Cosmogony, Jeans said:

Astronomy does not know whether or not life
is important in the scheme of things, but she begins to whisper that life
must necessarily be somewhat rare.

By the late 1920s, this opinion was shared by many astronomers. However,
in 1935, Henry Norris Russell raised what
would become fatal objections to the Jeans-Jeffreys hypothesis. He pointed
out that it was hard to see how a close stellar encounter could leave the
Sun, which is a thousand times more massive than the planets, with such
a tiny share of the solar system's angular
momentum. Furthermore, he could not understand how the planets could
condense out of hot material ejected from the Sun. The former objection
was put into stronger form by Russell himself in 1943, while the latter
was strengthened by Russell's student, Lyman Spitzer,
in 1939.